Photometers are widely employed for measuring the concentration of selected components or ingredients in a sample medium. For example, such instruments may be employed to test for the concentration of chlorine or other constituent chemicals in drinking or pool water. Typically, a photometer detects light transmitted through a sample medium and provides a voltage signal that is indicative of the intensity of that light. This value is logarithmically related to the concentration of the component being measured. A number of known devices employ analog instrumentation to calculate the concentration, which is typically displayed, in terms of absorption, by a vernier scale or similar means. Such a display is often difficult to read and interpolate, particularly at higher concentration levels where the scale is severely compressed due to its logrithmic nature. Moreover, because the test results are displayed in terms of absorption, the user must resort to reference tables in order to determine the concentration. This complicates the testing procedure considerably and particularly hinders field testing.
Alternatively, digital microprocessors may be employed to compute the concentration. However, such devices require software which must be programmed into the microprocessor thereby adding to the complexity and expense of the device.
Various indicator circuits employ LED's or Nixie tubes to display a measured voltage or concentration level. However, none of these circuits provides a simple and satisfactory means for instantaneously and accurately indicating incremental levels of concentration between broad concentration ranges; for example, where the broad range of concentration is measured in parts per million and, incremental measurements on the order of tenths of parts per million are required. Conventional indicator circuits, such as the circuit shown in U.S. Pat. No. 3,703,002, employ LED or Nixie tube indicators wherein a respective light is assigned to each incremental level. Thus, if a concentration range of 0-4 ppm is being tested and an accuracy of 0.1 ppm is required, 40 individual indicators must be employed. Such a display is inordinately large and difficult to decipher. It requires a photometer that is much too large for use in the field. Moreover, such an array of indicators is wholly impractical where a very high degree of accuracy is required.
Performing rapid and reliable null or zeroing calibrations is a further problem encountered by conventional photometers. Such calibrations are typically required to compensate for spurious readings caused by components in the sample medium other than the component being measured. Conventional means for accomplishing zeroing calibrations include manual adjustments and microprocessors. Manual adjustments are time consuming and subject to human error. Microprocessor calibration requires the complexity and expense of software. Although certain automatic zeroing circuits are known, these are typically utilized in "two cuvette" photometers wherein separate beams of light are transmitted through respective cuvettes containing a reference medium and a test medium. To date, no system is known for providing automatic and accurate zeroing calibration in a single cuvette photometer.